Charging high capacity batteries from 5V sources

Designers of portable electronics are challenged to create devices that do everything while running endlessly on a single battery charge. While it is impossible to fully meet this challenge, each successive generation of batteries at least comes closer to that goal. With devices now sporting large vibrant touch-sensitive displays, multicore CPU and graphics processors, and an assortment of wireless modems for high speed communications anywhere on the planet, high battery capacity is essential. Battery manufacturers have met the demand with light weight, compact cells with capacities to over 30 watt-hours.

While USB has become the dominant standard for device interconnect, synchronization and data exchange, its power delivery capabilities have not kept pace with battery demands. USB 2.0 allows a maximum 2.5W load, while USB 3.0 extends the limit to 4.5W. Even with perfect efficiency and all power going directly to the battery, a full charge cycle via USB would require overnight and then some. Though USB is not suitable as a primary power source for large capacity batteries, it still has great value as an opportunistic power source to charge when and where possible, and to prevent battery drain when the device is tethered to a traditional computer.

Linear has introduced the LTC4155, a monolithic switching battery charger that delivers 3.5A charge current efficiently in a compact PCB footprint. Figure 1 shows the required components in a typical application. The 2.25MHz switching frequency allows for a small inductor and bypass capacitors to minimize the overall PCB footprint.